I. Field of the Invention
The present invention relates generally to chisel inserts for rolling cone rock bits and more particularly to specifically shaped inserts for use on the drive rows of the rolling cones.
II. Description of the Prior Art
Rock bits using sintered tungsten carbide inserts with cutting tips having a generally wedge or chisel-shaped configuration are used for drilling soft and medium formations. Various configurations for wedge-shaped inserts are shown in U.S. Pat. No. 3,442,342. Inserts of this type have a pair of symmetrical flanks that converge to a rounded crest. The inserts are interferingly secured in holes drilled normal to the cutter surface.
In operation, as the cutter or cone rotates, the crest initially contacts the formation at a time when the longitudinal axis of the insert is non-perpendicular with respect to the hole bottom. Bending stresses are thus generated in the inserts, tending to cause breakage.
This is particularly true in the drive row of the cutters, the first row of inserts inboard of the gage row. Drive row inserts experience more chippage and breaking initiating at the corners of the insert crest.
To alleviate this breakage problem, the nose radius has been made larger across the entire crest length. Although such blunter inserts have been successful in reducing breakage, they have also functioned to reduce the rate of penetration of the bit.
The inner row inserts of U.S. Pat. No. 3,442,342 had slightly convex crests and flanks which intersected to enable the crest to have a uniform width. The patent further states that if the flanks were flat, the natural intersection with the crest would create a crest of non-uniform width, thin at the middle and flaring out to a larger dimension at each end. Such a crest was considered to be undesirable because if the center dimension were large enough to avoid breakage, the ends would also be so wide that the tip would be dull at those locations, and conversely, if the ends were thinned down to a sharp width, the center part of the crest would be so fragile as to invite early breakage.
Another prior art insert is shown in U.S. Pat. No. 4,254,840. This insert includes a cutting tip made primarily of a truncated cone having a hemi-spherical tip mounted thereon. The sides of the sphere are tangential to the conical surface. A pair of flats are then placed into the sides of the cutting tip.
The problem with such an insert is that the radius of the cutting tip is constant and relatively large thereby functioning to reduce the rate of penetration.